The present invention relates in general to screening machines, and in particular to an exchangeable planar screen for such machines including a screen frame arranged in a mounting support, a collecting floor and a fabric secured to the screening frame to act as a screening surface.
In screening cereal grain material to separate coarse foreign substances such as weeds, little cords, small pieces of rock and soil as well as fine foreign particles such as sand, small grains or broken grains, screening machines are employed which include screens having wide mesh fabric or so-called coarse screens as well as subsequent screens having fine meshed fabric, the so-called sand screens. Coarse foreign particles passing over the coarse screen are discharged through an outlet whereas the treated material passing through the coarse screen is fed to the subsequent sand screen or to a plurality of parallel connected sand screens arranged downstream of the flow of the treated material. The additional separation performed on the latter results in the provision of the cleaned product passing over the sand screens while fine foreign particles pass through the sand screens, the latter being guided to corresponding outlets whereby the clean product is supplied to a conveyor or if the separation of fine dust and light particles such as wheat seeds, shells, small straw particles and finely crushed grains is desired, the cleaned product is fed to a wind sifting machine where the fine foreign particles are sucked out or collected in containers and removed.
The above-described screening process is suitable for grain material having a larger grain size such as for example wheat, rye, barley, oats, corn or rice, but the aforementioned screen arrangement is unsuitable for treating grain material having fine grains such as for example rape seeds, sorghum or milo inasmuch as in the case of such fine grains the screening of fine foreign particles takes no effect. Accordingly, in treating such fine grains it is possible to eliminate only coarser foreign components while the fine grain material passes through the sand screen. Any coarser foreign particles which are larger than the grain material such as burdock in the case of rape seeds, pass over the screen. As a consequence, in the above-described screening machines the outlets for discharging fine foreign particles only now discharge the cleaned product whereas the outlets normally discharging the cleaned large grain material to a conveyor or to an air sifter would now discharge coarse foreign particles of smaller size. In order to avoid this situation and for adjusting conventional screening machines for the treatment of the aforementioned fine grain cereals, it has been already devised in practice to cover the sand screen with metal sheets or blind floors and to use the coarse screens or screen only. This temporary adjustment, however, is disadvantageous because of the fact that respective screening surfaces can be loaded to a certain limit only and therefore the height of the processed layers must not exceed a certain limit. Moreover, for screening fine foreign particles it is necessary to provide a larger sand screening surface than for coarse screening surfaces and this relatively large sand screening surface remains in the case of processing fine grain material completely unused.